Changing event suppression to signal suppression.

This is a resolution for ECOM-516.  This will change the new kwarg on CourseEnrollment.unenroll such that you can suppress the django signal, rather than the analytics events. This will avoid sending refund emails, but will preserve the analytics events associated with unenrollment. 

@mulby @feanil @wedaly @rocha 
Ideally I'd like to see a test in here that validates that the event *is* emitted

That feels like the type of test that would have failed before this change and would pass now with this change.